The present invention relates generally to the field of capacitors. In particular, the present invention relates to capacitors that can be embedded within a laminate dielectric material, such as in the manufacture of a printed circuit board.
Laminated printed circuit boards, as well as multichip modules, serve as support substrates for electronic components, such as integrated circuits, capacitors, resistors, inductors, and other components. Conventionally, discrete passive components, e.g. resistors, capacitors and inductors, are surface mounted to the printed circuit boards. Such surface-mounted discrete passive components can occupy a large amount of the real estate of a printed circuit board, thus limiting the space available for the mounting of active components, such as integrated circuits. The removal of passive components from the printed circuit board surface allows for increased density of active components, further miniaturization of the printed circuit board, increased computing power, reduced system noise and reduced noise sensitivity due to shortened leads. Embedding passive components within the laminated printed circuit board structure allows such components to be removed from the surface of the printed circuit board.
The capacitance density of a capacitor depends upon the thickness of the dielectric material, the dielectric constant of the dielectric material and the area of the electrodes. Decreasing the thickness of the dielectric material increases the capacitance density of the capacitor. The greater the capacitance density of a capacitor is, the smaller the required area is for the capacitor. Shrinking the area used by a capacitor also shrinks the amount of printed circuit board space required to use that capacitor. Thus, capacitors having thinner dielectric layers and accordingly smaller areas are desired. However, as the dielectric material layer becomes thinner, the topography of the underlying conductive substrate (electrode) becomes a significant consideration.
Conventional embeddable thin-film capacitor structures are typically prepared by the deposition of a dielectric material on a conductive substrate, such as a copper foil, with another conductive layer deposited on the dielectric material opposite the conductive substrate. The copper foils used to make these embeddable capacitors are those foils conventionally used in the printed circuit board industry.
Copper foils are typically produced by the electrodeposition of copper from a solution onto a rotating drum. The surface of the copper foil adjacent the drum is the smooth (or shiny) surface while the other surface has a much higher roughness (the matte side). The matte surface of the foil typically provides better adhesion to a substrate, e.g. a polymeric layer such as a photoresist or polymeric dielectric material such as glass reinforced epoxy. The topographic structure of metal foils are conventionally evaluated by looking at the roughness of the surface, that is the peak-to-valley distance.
Applying the capacitor dielectric material to the matte surface of the foil is thought to improve the adhesion between the capacitor dielectric material and the foil, as in the case of polymeric coatings. Thus, the rougher the surface, the better the adhesion of the capacitor dielectric material to the foil. However, very rough surfaces cause other problems when thin capacitor dielectric material layers are used. A rougher foil surface requires more capacitor dielectric material be deposited than a smoother foil surface to obtain a structure having a given capacitance density. The addition of more capacitor dielectric material requires additional deposition procedures as compared to those required for a smoother foil. Such additional procedures may add greatly to the cost of the process.
A foil surface that is too rough may cause uniformity problems in the capacitor dielectric material layer, leading to shorts. A foil surface that is too smooth may adversely affect the adhesion of the capacitor dielectric material to the foil. What is desired for thin-film capacitor manufacture is a metal foil having a sufficient smoothness to provide a thinner capacitor dielectric material layer having a desired capacitance uniformity while still providing sufficient adhesion to the capacitor dielectric material.
U.S. patent application Ser. No. 2003/0068517 (Andresakis et al.) discloses nickel coated copper foil for embedded passive devices. In this patent application, conventional copper foils are used. This patent application fails to appreciate the balance of the levels of roughness and smoothness required to prepare embeddable thin-film dielectric-containing capacitors without having an appreciable number of shorts.